90 abstracts: physical chemistry 



PHYSICAL CHEMISTRY.— r/ie utilization of diffusion processes in 

 the preparatio7i of pure substances. John Johnston. J. Am. Chem, 

 Soc, 36: 16-19. 1914. 

 Many slightly soluble substances, when formed by precipitation in 

 the ordinary way, are very fine-grained and consequently contain oc- 

 • eluded impurities which are not easy to get rid of; but by taking advan- 

 tage of the slowness of diffusion in liquids, one can secure very slow 

 precipitation, and in. this way prepare such substances in relatively 

 large crystals free from impurity. By this means, for instance, one can 

 readily obtain crystals of calcium hydroxide (Ca(0H)2) in the form of, 

 hexagonal prisnis 3 mm. long with the base 1 mm. thick, or crystals 

 of barium sulphate as much as 2 mm. long. J. J. 



PHYSICAL CHEMISTRY.— ne binary system MgO-SiO^. N. L. 

 BowEN and Olaf Andeksen. Am. Jour. Sci. (4), 37: 487-500. 

 1914. 



Equilibrium in the binary system MgO-Si02 was studied by applying 

 the method of quenching. 



There are two compounds, the orthosilicate Mg2Si04 and the meta- 

 silicate MgSiOs, capable of existing in contact with liquid in the binary 

 system. The former crystallizes in a form corresponding with the min- 

 eral forsterite and the latter forms crystals similar to enstatite in most 

 properties, but of monoclinic symmetry, clino-enstatite. 



Clino-enstatite is the only stable form of MgSiOs encountered. It 

 has no true melting-point, but breaks up at 1557° C. (formerly consid- 

 ered the melting-point) into forsterite and liquid, and the temperature 

 must be raised to 1577° C. before complete solution of the forsterite 

 takes place. 



In an earlier Geophysical Laboratory publication, crystals termed 

 a-MgSiOs were described as a high-temperature form of magnesium 

 metasilicate. They were considered to be the product of inversion of 

 clino-enstatite (/S-MgSiOs), but the crystals described have now been 

 proved to be a product of the dissociation at 1557° C. and to be the 

 orthosilicate forsterite, not a form of the metasilicate. 



On account of the break-up of clino-enstatite into forsterite and 

 liquid there is no eutectic between the two compounds, and the liquids 

 show, on cooling, the partial or complete re-solution of forsterite at the 

 reaction-point, 1557° C, the liquid reacting with the forsterite crystals 

 to give clino-enstatite. 



A discussion is given of the geological significance of this resorption 



